When travelling in places where tipping is expected, I tip, regardless of my budget.

I give 15% - 20% for good or exemplary service, and 10% for middling service. For unspeakably deplorable service, I still tip... but I become truly miserly! My reasoning is that a 5% tip sends a stronger message than no tip at all. The extreme form of this is to leave exactly 1 cent, but I have never had the chutzpah to do this. (But I have only received really bad, no-good, hostile service once in my life.)

In some jurisdictions, the minimum wage for restaurant servers is less than the minimum wage for other workers, precisely because the government assumes that the income of servers will be supplemented by gratuities. So when you don't tip someone who is doing a good job, you are, in a sense, taking advantage of someone who, due to bad luck, lives in a jurisdiction that has regressive labour laws.

Better that you work an extra few days so you'll have enough money to tip restaurant workers who provide you with good service.